1. Field of the Invention
The present invention relates to a surface light source device and an image display unit and, in particular, to a technology for reducing the thickness of each of a surface light source device and an image display unit and improving the functionality thereof by disposing first and second light sources at predetermined locations relative to a light control member.
2. Description of the Related Art
Recently, surface light source devices that illuminate using a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED) have been widely used. For example, such surface light source devices are used for lighting equipment that employs a light source for direct lighting. In addition, the surface light source devices are used for image display units that employ a light source for backlighting (e.g., television receivers and personal computers).
A liquid crystal display unit is one of such image display units. A liquid crystal display unit includes, for example, a liquid crystal panel that displays an image thereon.
In a liquid crystal display unit, since a liquid crystal panel is not a light-emitting device, a surface light source device is provided. The surface light source device includes a light source that emits light from the back side of the liquid crystal panel. Accordingly, the surface light source device is used for a backlight device that emits light to a liquid crystal panel from the back side of the liquid crystal panel.
Surface light source devices can be categorized into two types: a side edge type and a direct light type. Surface light source devices of a side edge type include a light guiding member. A light source is disposed on the side of the light guiding member. Light emitted from the light source is directed by the light guiding member towards a display panel placed in a predetermined direction. In contrast, in surface light source devices of a direct light type, a light source is disposed on the back side of a display panel. Light is directly emitted from the light source to the display panel.
For liquid crystal display units used for television receivers and large-screen liquid crystal display units, surface light source devices of a direct light type are used in order to obtain a bright image and maintain a bright screen for a long time.
In general, surface light source devices used for liquid crystal display units include a light diffusing member (a light diffusing plate or a light diffusing sheet) disposed so as to face the back surface of a liquid crystal panel and a light reflecting member disposed on the back side of the light diffusing member.
For example, a surface light source device include a plurality of fluorescent lamps (e.g., CCFLs) serving as a light source. The fluorescent lamps are arranged in a predetermined direction. A light diffusing member is disposed between a display panel and the fluorescent lamps. The light diffusing member diffuses light emitted from the fluorescent lamps so that the display panel is uniformly illuminated with the light. Thus, users do not visually recognize the external shapes of the fluorescent lamps, and therefore, the presence of the fluorescent lamps.
In addition, the light diffusing member uniformly emits light output from the light source to the liquid crystal panel. In this way, the luminance of the display screen of the liquid crystal panel can be maintained uniform.
In the liquid crystal display units, after light is emitted from a light source, the emitted light is diffused by the light diffusing member and is emitted to a liquid crystal panel that displays an image from the back side of the liquid crystal panel. At that time, the light is emitted from the light source towards the light reflecting member as well as towards the light diffusing member. The light emitted towards the light reflecting member is reflected by the light reflecting member and is then made incident on the light diffusing member. Thereafter, the light is diffused by the light diffusing member, and the diffuse light is emitted to the liquid crystal panel.
An example of an existing image display unit, including a surface light source device serving as a backlight device of a direct light type, is described next with reference to FIG. 13.
An image display unit a includes a casing (not shown). A variety of associated components are disposed inside the casing.
The casing has a box-like shape having a small thickness in the forward-backward direction. The casing has an opening on the front side thereof. A display panel b is disposed inside the casing so as to close the opening. A driving circuit c is disposed, for example, beneath the display panel b and is electrically connected to the display panel b. The driving circuit c drives and controls the display panel b.
A surface light source device d is disposed inside the casing. The surface light source device d includes a light reflecting member e, fluorescent lamps f, f, . . . , and a light diffusing member g in this order from the back side.
For example, the light reflecting member e is formed from a white or silver resin or metal.
For example, the fluorescent lamps f, f, . . . are arranged at equal intervals in the vertical direction. The fluorescent lamps f, f, . . . are CCFLs, for example. Each of the fluorescent lamps f, f, . . . has a horizontally long cylinder shape. Each of the fluorescent lamps f, f, . . . includes a glass tube having a discharge space therein. A discharge medium is enclosed inside the glass tube.
The light diffusing member g diffuses light rays emitted from the fluorescent lamps f, f, . . . and emits the diffuse light rays to the display panel b. For example, the light diffusing member g is formed from a diffusing plate or a diffusing sheet. For example, the material of the light diffusing member g is a transparent or translucent white resin. More specifically, the material of the light diffusing member g may be a glass, a mixture of two mutually immiscible types of resin, a transparent resin containing a light diffusing material dispersed therein, or a transparent resin. The light diffusing member g has a light entrance surface and a light exit surface. The light entrance surface allows light rays emitted from the fluorescent lamps f, f, . . . to enter therethrough. The light exit surface diffuses the light rays entering through the light entrance surface and emits the diffuse light.
In recent years, the demand for reducing the thickness of such image display units has been increasing. However, if the distance between a light diffusing member and fluorescent lamps is decreased in order to reduce the thickness of the image display unit, the functionality of the light diffusing member is degraded, and therefore, the presence of the fluorescent lamps may be recognized in the form of the shapes of individual lamps by the users.
Therefore, some existing image display units provide a thin plate-like light control member called a “prism array” between a light source and a light diffusing member so as to increase the light diffusing function thereof. In this way, the presence of the fluorescent lamps is rarely recognized in the form of the shapes of individual lamps by the users (refer to, for example, Japanese Unexamined Patent Application Publication No. 63-110422).
In addition, a demand for increasing the performance of image display units (e.g., the image quality) has been increasing. In order to improve the performance of image display units, some existing image display units have a second light source, having a characteristic different from that of a fluorescent lamp, between fluorescent lamps. An example of the second light source is an LED (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-139876).
In the image display unit described in Japanese Unexamined Patent Application Publication No. 2004-139876, an LED that is usable at low power is used for a dark screen. In contrast, a fluorescent lamp is used for a bright screen while high power is being provided to the fluorescent lamp. In this way, a wide light control range is obtained, and therefore, the performance can be improved at low cost. In addition, since the wavelengths of light rays emitted from the fluorescent lamp and the LED are different, the color control range can be increased.